coal; but in special cases anthracite coal is used. Soft coal is converted into gas by burning it in a thick fire. Air is blown in beneath the grate, and a jet of steam is also admitted to keep down
FIG. IX-A.óWATER SEAL PRODUCER.
the temperature. Within the last few years the water seal producer has been generally adopted. Many different forms have been used, but the main principles of the construction are illustrated in Fig. IX-A. The space below the water level is full of ashes, which, can be removed without interfering with the operation of the producer. The ashes will fill the room for one or two feet above the water line. Above this will be glowing carbon, and the air as it goes up forms carbonic acid (C02)3 and this rising through the bed of coal absorbs more carbon and becomes carbonic oxide (CO), but this action is never complete, and some carbonic acid passes through the fire unchanged. With a hot deep fire free from cavities the gas may contain as low as 2.5 per cent, by volume of C02, but if the fire be thin or riddled with holes there may be as much as 10 per cent. In the "zone of combustion" the steam is broken up by the carbon with formation of hydrogen and carbonic oxide, but, as in the reduction of carbonic acid, this reaction is never perfect and some steam goes through unaltered. The best decomposition is attairied in a hot fire, but this is Just the condition that is not desirable on account of the formation of clinkers. On the other hand, if- the supply of steam be increased indefinitely ihe fire will get colder